There are switching-fields with a coupling path formed with switching matrices containing thyristors suitable for use in stored program controlled telephone exchanges. Such is described for instance in the French patent specification No. 2 357 132. This switching-field is characterized in that transformers are connected to the input of the first stage and to the output of the last stage, and two different voltage sources are connected through resistors to the centers of the secondary and primary coils of the transformers to supply the holding current necessary for maintaining the connection already set up, while upon disconnection a third, opposed polarity voltage source is connected to the outputs of the last stage, which reduces the holding current of the connection to zero. Further characteristics of the switching-field described in the quoted specification is that the circuits carrying out the allocation and switching of the connections are formed stage by stage and are connected through logical circuits to the stored program control device.
As a result of the transformer connected to the output of the last stage, three transformers will be used in all such connections which are set up between the lines connected to the transformers joined to the inputs of the first stage. This entails the disadvantage that in case of transformers characterized with identical parameters, the transmission parameters of the connection are worse than in case when only two transformers are used. Such switching is also possible in the switching-field according to the quoted patent specification, but in this case only the inputs of the first stage can be connected with the outputs of the last stage, in other words the number of stages participating in the connection is less, which results in the increase of the loss given by the internal blocking of the switching-field. The incorporation of the third transformer increases the specific volume of the switching-field characteristic to one connection, which represents a further disadvantage. Further drawback of the quoted solution is that the first voltage source necessary to maintain the holding current is connected through a resistor to the center of the secondary coils of the transformers joined to the inputs of the first stage. Upon switching on--when two or more coupling paths are connected to a given input of the first stage--the holding currents of these coupling paths would be of reduced value, which detrimentally influences the dynamic performances and the insensitivity against disturbances. The fact that the polarity of the third voltage source is opposite to that of the first two voltage sources represents further disadvantage in many cases. The allocating and switching-on circuits, utilized stage by stage, unnecessarily increase the number of units built into the switching-field, because the inner information content of a multi-stage switching-field with coupling path is not utilized. The same disadvantage arises in respect of the adjoining logical circuits too, which unnecessarily increase the number of wires running towards the stored program control device. All these finally increase the power consumption of the switching-field. At the same time the direct connection of the logical circuits to the stored program control device means that the control device has to suspend all other activities at setting up or clearing each connection.